1. Field of the Invention
The invention relates to an exposure device for scanning a light beam on a surface of a latent image carrier, such as a photosensitive member, along a main scan direction. The invention also relates to an image forming apparatus equipped with the exposure device.
2. Description of the Related Art
Conventionally, there have been known image forming apparatuses, such as printers, copiers and facsimiles, which form an image by scanning a light beam on the latent image carrier, such as a photosensitive member, according to image data. The exposure unit of such an image forming apparatus scans a light beam, modulated based on the image data, on the surface of the latent image carrier such as the photosensitive member along a main scan direction by means of an optical scanning system, whereby a latent image corresponding to the image data is formed on the latent image carrier. After the latent image is developed into a toner image, the resultant toner image is transferred to a sheet such as a transfer sheet, plain paper sheet or copy sheet. As stated in Japanese Unexamined Patent Publication No. 2002-296531 (hereinafter referred to as “Patent Document 1”), the optical scanning system heretofore known to the art employs a polygon mirror as a light deflector.
In the first conventional apparatus, the photosensitive member as the latent image carrier is rotated at a constant speed in a predetermined direction (subscan direction). A light beam emitted from a light source such as a semiconductor laser is shaped into a collimated beam of a suitable size by means of a collimator lens and then applied to the polygon mirror. Thus, the beam is deflected by the polygon mirror so as to be scanned along the main scan direction. The scanned beam is focused on the photosensitive member via an f-θ lens comprised of two scanning lenses.
By the way, it is impossible to completely eliminate the configuration errors of the light deflector, such as a polygon mirror or pivotal mirror. For instance, the polygon mirror has a plurality of deflector mirror planes, each of which reflects the light beam. If the deflector mirror plane has an inclination relative to a rotary axis of the polygon mirror, a so-called tilted deflection plane results. Hence, a scan position of the light beam on the photosensitive member is deviated from a reference scan position with respect to the subscan direction so that the image quality is degraded. The invention disclosed in Patent Document 1 does not give adequate consideration to the configuration errors of the light deflector and improvement in the image quality is wanted. Incidentally, the reference scan position is previously defined by design, representing a position on which the light beam is to be scanned.
As conventionally known in the art, an effective measure for correcting the tilted deflection plane is to arrange a pair of cylindrical lenses powered only in the subscan direction at places forwardly and rearwardly of the light deflector. Such an arrangement establishes an optically conjugated relationship between the deflector mirror plane and the surface of the photosensitive member with respect to the subscan direction. Thus, a focusing position on the photosensitive member is not displaced even if the deflector mirror plane has an inclination.
Unfortunately however, the addition of the cylindrical lenses results in an increased number of components and hence, the apparatus is increased in costs. The increased number of optical components necessitates the increase in the size of the optical scanning system, which constitutes a major obstacle to the compact design of the image forming apparatuses. In addition, there is another problem of cumbersome optical adjustment.
Even though the aforementioned tilted deflection plane is corrected, component tolerances and assembly errors are unavoidable. In this connection, there has been a strong demand for a method which permits the scan position of the light beam in the subscan direction to be readily registered with the reference scan position without relying upon re-assembly and re-adjustment of the optical scanning system in the final adjustment stage following product assembly.
During the operation of the image forming apparatus, furthermore, the scan position of the light beam in the subscan direction may be shifted from the reference scan position because of changes in the operating environment including temperature, moisture and such, displacement of the optical component due to vibrations or age-related changes.
Such a problem is also common to image forming apparatuses for forming a color image. In addition, the color-image forming apparatus has the following problem. As the image forming apparatus of this type, there has conventionally been known one wherein a photosensitive member, an exposure unit and a developing unit are dedicated to each of four different colors including, for example, yellow, magenta, cyan and black (see, for example, Japanese Unexamined Patent Publication No. 8-62920). In this second conventional apparatus, individual images for the respective color components are formed on the photosensitive member as follows. On a per-color basis, a light source of an exposure unit is controlled based on image data representative of an image of each color component. In the meantime, a light beam from the light source is scanned on the surface of the photosensitive member along the main scan direction by means of the optical scanning system of the exposure unit. Thus, a latent image corresponding to the image data of each color component is formed on the photosensitive member.
There has also conventionally been known another apparatus wherein four light sources are provided whereas a common polygon mirror deflects light beams from the individual light sources for scanning the beams in the main scan direction (see, for example, Japanese Unexamined Patent Publication No. 2001-296492). In the third conventional apparatus, four scanned beams from the polygon mirror are respectively guided to four photosensitive members by means of respective groups of reflection mirrors so as to form respective latent images.
By the way, the second conventional apparatus has a requirement that the exposure units be provided in association with the individual color components. This not only results in the increased apparatus costs but also constitutes a major obstacle to the downsizing of the apparatus. The third conventional apparatus is more advantageous than the second conventional apparatus in terms of the apparatus costs and the downsizing thereof because the polygon mirror is shared. Similarly to the second conventional apparatus, however, the third conventional apparatus requires the provision of the same number of light sources as the color components. Thus, the third conventional apparatus still has a room for improvement in terms of the downsizing of the apparatus and such.
For the purpose of achieving a higher monochromatic printing speed than a color printing speed, the third conventional apparatus has an arrangement wherein the light beams from the light sources are arranged in close adjacency to one another and on a line along the subscan direction (direction substantially perpendicular to the main scan direction of the light beams). Furthermore, an additional light path switching portion is interposed between the light sources and the polygon mirror such as to switch the light path of the light beams between the color printing mode and the monochromatic printing mode. Therefore, the third conventional apparatus is increased in costs because of the provision of the light path switching portion for varying the printing speed between the monochromatic printing and the color printing. In addition, there exists a need for exactly adjusting the relation between the four light sources, the light path switching portion and the polygon mirror, which poses a serious problem associated with adjustment operation.
On the other hand, an image forming apparatus of a tandem system operates as follows. That is, latent images corresponding to image data items of four colors are formed on a yellow-, magenta-, cyan- and black-photosensitive member, respectively and the resultant latent images are used to form a color image. The image forming apparatus is also adapted to perform a monochromatic printing operation in addition to the color printing operation. Specifically, in the monochromatic printing operation, the optical scanning system scans only the light beam from a light source corresponding to a single color (black) so as to form a latent image on the black photosensitive drum. In this case, the optical scanning system does not scan the light beam on the photosensitive members for the other three colors (yellow, magenta and cyan). Accordingly, the latent image for monochromatic printing only need be formed on the black-photosensitive member. However, the formation of the latent image on the black-photosensitive member takes as much time as the formation of the images for color printing.
Such a problem is not limited to the image forming apparatus wherein the latent images corresponding to the image data items of the four colors are formed on the four photosensitive members, respectively, and the resultant latent images are used to form a color image. The problem is also common to image forming apparatuses wherein latent images of N colors (N denoting a natural number of N≧2) are formed on N latent image carriers such as photosensitive members, respectively, and the resultant latent images are used to form the color image. In other words, an important problem for such an image forming apparatus of the tandem system is to reduce time required for forming the image with colors limited to (N−1) or less.
Besides the aforementioned apparatus of the tandem system, the color image forming apparatus further includes an apparatus of a so-called 4-cycle system. The fourth conventional apparatus is exemplified by one disclosed in Japanese Unexamined Patent Publication No. 2001-235924. If the photosensitive member or the transfer medium is fluctuated in speed, the color-image forming apparatuses of either system encounter registration deviation between a toner image being transferred and a toner image previously transferred to the transfer medium. Even though the photosensitive member or the like is not fluctuated in speed, another drawback occurs in a case where the rotation period of the photosensitive member rotated in the subscan direction is not integral multiples of the scan period of the light beam scanned in the main scan direction. That is, the toner images of the individual colors may be out of registration by one scan stroke in maximum with respect to the subscan direction. Heretofore, there have been proposed techniques for controlling the rotational speeds of the photosensitive member and the transfer medium in order to reduce the incidence of such a registration deviation.
In cases, however, changing the rotational speed of the photosensitive member or the like may cause a problem. Specifically, if the rotational speed is changed at timing of forming the latent image by scanning the light beam on the photosensitive member or of transferring the toner image from the photosensitive member to the transfer medium, the latent image or the toner image may be adversely affected. This will result in the degraded image quality. In order to eliminate the registration deviation by changing the rotational speed, the photosensitive member or the like must be frequently changed in the rotational speed thereof during the image forming operation. This leads to an instable running speed of the photosensitive member or the like and hence, the degraded image quality will result.